Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers, having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then collects residual toner not transferred and remaining on the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
Typically, the fixing device may include a fixing roller heated by a heater, and a pressing roller pressed against the fixing roller to form a nip therebetween through which the recording medium passes. As a recording medium bearing a toner image passes through the nip, the fixing roller and the pressing roller apply heat and pressure to the recording medium to melt and fix the toner image on the recording medium. Thereafter, the recording medium bearing the fixed toner image is discharged from the nip.
Ordinarily, after finishing one job the fixing device enters a standby state in which the pressing roller remains pressed against the fixing roller while the heater heats the fixing roller to prepare for the next print job. However, if the pressing roller is constantly pressed against the fixing roller, an elastic layer of both the pressing roller and the fixing roller may be deformed permanently into a bow-like shape, generating a gap between the pressing roller and the fixing roller. As a result, the pressing roller and the fixing roller may not convey the recording medium precisely, forming a faulty toner image on the recording medium and generating noise.
To address this problem, the fixing device may further include a moving assembly that moves the pressing roller with respect to the fixing roller so as to alternately press the pressing roller against the fixing roller and separate the pressing roller from the fixing roller. For example, the moving assembly separates the pressing roller from the fixing roller when the image forming apparatus finishes a print job, thus minimizing deformation of the pressing roller and the fixing roller due to pressure therebetween at the nip.
Two control methods are known for control of the moving assembly. A first method is to separate the pressing roller from the fixing roller immediately after a recording medium bearing a fixed toner image is discharged from the nip, minimizing the amount of time the pressing roller presses against the fixing roller. A second method is to separate the pressing roller from the fixing roller when the image forming apparatus does not receive the next print job within a predetermined time period after finishing the previous print job.
The first method has advantages in that the minimized time for which the pressing roller presses against the fixing roller reduces not only deformation of the rollers but also consumption of power because the standby state is omitted. However, the first method has a drawback in that, since the pressing roller separates from the fixing roller whenever the print job is finished, it increases the frequency of separating the pressing roller from the fixing roller, which generates considerable noise due to a spring installed in the moving assembly and generates excess wear on the pressing roller and the fixing roller.
By contrast, the second method has an advantage in that the frequency of separating the pressing roller from the fixing roller is decreased. However, the second method has a drawback in that the pressing roller continues being pressed against the fixing roller while the heater heats the fixing roller for a predetermined time period in the standby state even though the image forming apparatus does not receive the next print job, thus wasting power.
Accordingly, there is a need for a technology that achieves the optimum balance between minimization of noise significant in the first method and minimization of power consumption significant in the second method.